A synchronous motor is a motor of which the speed can be controlled based on a frequency applied from a motor control unit. Generally, a synchronous motor is started after aligning a rotor of the synchronous motor. Then, the rotor speed of the synchronous motor is increased by gradually increasing a control frequency.
FIG. 10 illustrates coordinates system for explaining a method of aligning a motor rotor.
Referring to FIG. 10, U, V, and W denote axes passing through centers of stator coils of a motor; d denotes an axis defined in a main flux direction that varies with a rotor of the motor; and q denotes an axis angled 90 degrees in electric angle from the d-axis in a forward direction. N an S denote the positions of N and S poles of magnets of the rotor, respectively. It is configured such that when a current is applied in the direction of the d-axis, the motor stops, and when a current is applied in the direction of the q-axis, the motor rotates. In the above, the UVW coordinate system is a stationary coordinate system, and the d-q coordinate system is a rotating coordinate system.
In the above-defined coordinate systems, when it is intended to forcibly align the motor rotor before starting the motor, an alignment current can be applied as a current vector defined in the d-axis direction. For example, a pulse current vector is applied a plurality of times in the direction of the d-axis.
A load can be applied to an object connected to a rotation shaft of the motor, or the object can be in motion. For example, a fan of an outdoor unit of an air conditioner can be rotated by a wind. In this case, a fan driving motor connected to the fan is started in a state where the fan is already rotated in a forward or reverse direction.
Thus, in this case, the fan may wobble or vibrate when an alignment current is momentarily applied in the d-axis direction. Furthermore, due to the rotational inertia of the fan, if the alignment current is not correspondingly great, a rotor of the fan driving motor is not aligned to an initial position. Moreover, due to the increase of the alignment current, power consumption increases.